The present invention relates to novel bis-poly(ethylene oxide) oligomeric surfactants, to a process for their production and to their use as antistats for photographic coatings.
It is known in the art to include a surfactant in photographic coatings in order to impart antistatic properties. The use of nonionic fluorosurfactants based on poly(ethylene oxide) is disclosed in U.S. Pat. Nos. 4,582,781 and 4,610,955. A typical nonionic fluorosurfactant of this kind is commercially available under the trade name Zonyl FSN and has the following linear chemical structure
CxF2x+1(CH2)yxe2x80x94(CH2CH2O) zxe2x80x94OH
in which x varies from approximately 4 to 14 but is about 8 on average; y is 2; and z is a distribution of numbers with an average of about 10.
However, it would be an advance in the art if surfactants could be provided that were easier to dissolve in water than the nonionic fluorosurfactants typified by Zonyl FSN and yet which exhibited more surface activity or hydrophobicity.
The present invention, in one of its aspects, provides the compounds of the general formula 
in which
R is an alkyl group with 10-20 carbon atoms, an alkenyl group with 10-20 carbon atoms or a fluoroalkyl or fluoroalkenyl group with f carbon atoms that each bear at least one fluorine atom and g carbon atoms that bear only hydrogen atoms, in which 1.5f+g=10-20 and g may optionally be zero,
Rxe2x80x2 is an alkyl group with 1 to 4 carbon atoms,
L is a linking group, a linking atom or a chemical bond, and
p is at least 2, q is at least 2 and p+q=from 4 to 100.
In another of its aspects, the present invention also provides a process for the production of an optionally substituted alkyl poly(ethylene oxide), which process comprises the steps of (1) reacting maleic anhydride with a poly(ethylene glycol) alkyl ether of the general formula
Rxe2x80x2O(CH2CH2O)pHxe2x80x83xe2x80x83(2)
and/or with a poly(ethylene glycol) alkyl ether of the general formula
Rxe2x80x2O(CH2CH2O)qHxe2x80x83xe2x80x83(3)
in which
Rxe2x80x2, p and q have the meanings stated above,
and (2) reacting the product of step (1) with a compound of the general formula
RLxe2x80x2Hxe2x80x83xe2x80x83(4)
xe2x80x83in which
R has the meaning stated above, and
Lxe2x80x2 is a linking group or linking atom.
In a further aspect, the present invention also provides a process for the production of an optionally substituted alkyl poly(ethylene oxide) in which a compound of the general formula 
in which
R and L have the meanings stated above is reacted with a compound of the general formula
Rxe2x80x2O(CH2CH2O)pHxe2x80x83xe2x80x83(2)
and/or with a compound of the general formula
Rxe2x80x2O(CH2CH2O)qHxe2x80x83xe2x80x83(3)
in which
Rxe2x80x2, p and q have the meanings stated above.
In yet another of its aspects, the present invention further provides a photographic element comprising a substrate and a coating on the substrate, wherein the coating contains a compound of the formula (1) above or a compound that has been prepared by a process of this invention. The present invention also provides a layer of gelatin that contains a compound of the formula (1) or a compound prepared by a process of this invention.
The compounds of the present invention, which may be classified as alkyl poly(ethylene oxides), are represented by the general structure (1) given above. In that structure, R is preferably an alkyl group of the formula CkH2k+1, an alkenyl group of the formula CjH2jxe2x88x921 or a fluoroalkyl group of the formula CnF2n+1xe2x80x94(CH2)m, or a corresponding fluoroalkenyl group, wherein k, j or (n+m) are equivalent to 10 to 20 carbon atoms (e.g. CH3 and CH2), given the premise that 1 fluorinated carbon (e.g. CF3 or CF2) equates to 1.5 hydrocarbon carbons. In certain preferred embodiments, R is selected from alkyl groups having from 10 to 20 carbon atoms, alkenyl groups having from 10 to 20 carbon atoms and fluoroalkyl groups of the general formula CnF2n+1 (CH2)m wherein the sum of (1.5n)+m is from 10 to 20. The definition of n and m is based on the premise that one fluorinated carbon (e.g. CF3 or CF2) equates to 1.5 hydrocarbon carbons (e.g. CH3 or CH2) in hydrophobic terms (from comparisons of critical micelle concentrations between alkyl and fluoroalkyl surfactants containing the same hydrophilic group; see E. Kissa, Fluorinated Suactants, Surfactant Science Series, Vol.50, Marcel Dekker (1994), 227-228). Preferably, a fluoroalkyl or fluoroalkenyl group R will be attached to the succinic acid moiety by a fluorinated carbon atom (as, for instance, when m=zero) only when L is a single chemical bond.
Although, in principle, the hydrophobic group R may be branched, it is currently preferred that R be a straight-chain group, because systems with straight hydrophobic tails are cheaper to produce. The hydrophobic group R is linked to the succinate moiety by L, which may be a linking group, such as xe2x80x94NHxe2x80x94, or linking atom, such as xe2x80x94Sxe2x80x94, or simply a chemical bond, in particular a single or sigma ("sgr") chemical bond.
Preferably, Rxe2x80x2 denotes a methyl, ethyl, n-propyl or isopropyl group. More preferably it denotes a methyl group. The Rxe2x80x2 groups in formulae (2) and (3) may be the same or different.
In the general structure (1), p and q, which may be the same or different, may vary but preferably p+q =from 8 to 50, more preferably from 8 to 32. In certain preferred embodiments; p and q are selected so as to balance the hydrophobic group in such a manner as to produce a soluble surfactant with a low critical micelle concentration, typically of the order of 10xe2x88x923M (molar) or less, preferably 10xe2x88x924 M (molar) or less. In certain preferred embodiments, p and q denote the same number.
It will be appreciated, however, that the general formula (I) extends not only to individual compounds of the stated structure but also to mixtures of two or more compounds having the stated structure. In such mixtures, p and q may refer to average values.
As noted above, the compounds of the present invention may be prepared by a process in which maleic anhydride is reacted with a poly(ethylene glycol) alkyl ether, and the resultant polyethylene glycol diester of maleic acid is thereafter reacted with a compound of the general formula RLxe2x80x2H in which R and Lxe2x80x2 are as defined above. The reaction of the maleic anhydride with the poly(ethylene glycol) alkyl ether is generally carried out in the presence of an acid catalyst, for example sulphuric acid. The diester, which constitutes an intermediate product, may be isolated prior to further reaction. The diester intermediate product is generally reacted with the compound RLxe2x80x2H in the presence of a basic catalyst, for example potassium carbonate. In certain preferred embodiments, the compound of the formula RLxe2x80x2H is a thiol of the formula RSH, in which case the end product may be classified as an alkyl-, alkenyl-, or fluoroalkyl- or fluoroalkenyl-thiosuccinic acid, polyethylene glycol diester.
In either of the preparative processes the poly(ethylene glycol) alkyl ether may be a single compound but will usually be a mixture of such compounds having different ethoxy chain lengths. Thus the resultant product of the formula (I) will contain compounds having different chain lengths in the two ethoxy chains. It is possible to introduce the different ether groups sequentially: the first ethoxylate can be added to the anhydride to give a half ester, the remaining chain length can then be transformed into the acid chloride and the second chain can then be added under basic conditions; this may be useful if p and q are to be different. Another possibility is to fractionate a mixture of products of the formula (1).
The synthesis of fluoroalkyl, alkyl, fluoroalkenyl or alkenyl derivatives according to the first preparative process may be illustrated by the following reaction scheme (Scheme 1): 
In reaction Scheme 1, p =4-16 and R has the meaning stated above in respect of formula (1), for example C6F13CH2CH2, C8F17CH2CH2 or C14H29.
Compounds according to the present invention may also be prepared, again as noted above, by a process in which an RL- substituted, e.g. an alkyl-, fluoroalkyl-, alkenyl- or fluoroalkenyl-substituted, succinic anhydride is reacted with an alkyl end-capped polyethylene glycol. This alternative preparative method provides, in particular, a way of producing compounds in which L is a single chemical bond and may be of particular interest in the production of compounds wherein R is an alkenyl group because alkenyl-substituted succinic anhydrides are commercially available in bulk. Of course, the other substituted succinic anhydrides are readily preparable by the person skilled in the art. The ethoxylated starting materials are available commercially or can be readily prepared by conventional chemical syntheses. This second preparative method may be illustrated by the following reaction scheme (Scheme 2): 
In reaction Scheme 2, R has the meaning stated above in respect of Formula (I), for example C18H37 or C18H35, and p=4-15. As indicated, the reaction is generally carried in the presence of an acid.
The compounds of the present invention, which may broadly be described as nonionic surfactants based on alkyl or alkenyl hydrophobic groups, which optionally can be at least partially fluorinated, and on two separately linked poly(ethylene oxide) hydrophilic groups, have been found to exhibit improved solubility and surface activity, compared with linearly structured poly(ethylene oxide)-based surfactants such as Zonyl FSN. Here, the notion of xe2x80x9cimproved surface activityxe2x80x9d is understood to mean lower critical micelle concentration (CMC). Although the applicant does not wish to be bound by theory, it is believed that the improved solubility is due to the structuring of the hydrophilic group as two independently linked poly(ethylene oxide) chains, which confer a Y shape to the resulting molecule. This is believed to increase, in effect, the cross-sectional area occupied by the hydrophilic group, which encourages the formation of simple spherical micellar systems, which are easier to solubilise than systems forming rod-like or lamellar phases.
The compounds of the present invention may be used as antistatic agents in photographic products, for example coatings on substrates in photographic elements. Thus, the compounds of the invention have been found to be particularly useful for promoting surface conductivity in dried coated layers of aqueous gelatin and, as such, are useful for controlling electrostatic charging in photographic products. In such applications, the present invention may provide the advantages of better surface electrical conductivity and greater ease of dissolution than nonionic surfactants of simpler structure that are known and used in the art, such as the fluoroalkyl poly(ethylene oxide) compounds of linear chemical structure.
When present in a coating a fluorocarbon surfactant will tend to give rise to a negative electric charge upon impact of the coating against a stainless steel reference surface, whereas a hydrocarbon surfactant will tend to give rise to a positive charge. Such static electrical charges can be troublesome in various applications. Thus, in certain embodiments, it may prove beneficial to employ a mixture of at least one fluorocarbon compound of the present invention (e.g. one in which R=CnF2n+1 (CH2)m) with at least one hydrocarbon compound of the present invention (R=alkyl or alkenyl) in order to balance out (or largely so) such triboelectric charges.
In certain embodiments, the compounds of the present invention may be used in conjunction with a low lattice energy salt, for example an inorganic tetrafluoroborate, perfluoroalkyl carboxylate, hexafluorophosphate or perfluoroalkyl sulphonate. Preferred are alkali metal tetrafluoroborates, trifluoroacetates, perfluorobutanoates, hexafluorophosphates, perfluorobutanesulphonates and trifluoromethanesulphonates. Such salts are discussed in U.S. Pat. No. 4,610,955 (Chen et al) and U.S. Pat. No. 4,582,781 (Chen et al), whose teaching is incorporated herein by reference. In certain especially preferred embodiments, the salt is an alkali metal triflate, in particular lithium triflate, CF3SO3Li.